[O] processing lines before display to agenda

[Joe Corneli]

I would find it quite useful to have a variable similar to
`org-agenda-prefix-format' that would apply changes to the *contents*,
rather than to the *prefix* of agenda items.

The nice thing about `org-agenda-prefix-format' is that it allows
different formatting for different agenda buffers.  Similarly, it would
be useful to be able to supply different functions to alter the agenda
items themselves, depending on the context.

I was thinking that in theory this could be done by advising
`org-agenda-format-item', but I'm not sure how to get the advice to
apply in a conditional way depending on the agenda.

There was a similar discussion related to the specific case of removing
TODO keywords from certain agendas,
https://lists.gnu.org/archive/html/emacs-orgmode/2010-09/msg00810.html

So, for example: to hide TODO keywords from the agenda:

(setq org-agenda-custom-commands
      '(("a" agenda ""
         ((org-agenda-todo-keyword-format "")))))

But what if, for example, I would like to remove priorities cookies as
well?